This invention relates to continuous casting machines for continuously casting metal ingot, strip, slab or bars directly from molten metal in a casting region defined between spaced portions of a pair of revolving, flexible, endless casting belts which are moved along with the metal being cast, often called twin-belt casting machines or twin-belt casters.
The invention is described as embodied in the structure and operation of twin-belt casting machines in which the molten metal is fed into a casting region between opposed, portions of a pair of moving, flexible belts. The moving belts confine the molten metal between them and carry the metal along as it solidifies into a bar, strip, slab, or ingot, hereinafter called the "cast product" or "product being cast" or similar words. Back-up means, usually rollers having narrow circumferential ridges or fins support and guide the belts while holding them accurately positioned and aligned as they move along so as to produce the cast metal product.
These back-up rollers are positioned across the machine carriages so as to roll passively when the casting belt grazes each of them under pressure of the head of molten metal and/or the weight of the metal. Their circumferential fins permit the passage of cooling liquid along the respective casting belt without notably impeding heat transfer themselves. The fins have often been made separately from the roller shafts, but in current machines the fins and shafts are now often made integrally as one piece of metal. Vast quantities of heat liberated by the molten metal as it solidifies are withdrawn through the portions of the two belts which are adjacent to the metal being cast. This large amount of heat is withdrawn by cooling the reverse surfaces of the belts by means of the rapidly moving liquid coolant traveling along these surfaces. The edges of the molten product are contained between a spaced pair of side dams in the form of a plurality of blocks strung together on flexible metal straps to form a pair of endless flexible assemblies suitable for containing the molten metal as it solidifies.
Background information on twin-belt casting machines will be found in U.S. Patents:
______________________________________ U.S. Pat. No. Inventor(s) ______________________________________ 2,640,235 Hazelett 2,904,860 Hazelett 3,036,348 Hazelett et al *3,123,874 *Division of No. 3,036,348 *3,142,873 " *3,228,072 " 3,041,686 Hazelett et al 3,167,830 " 3,310,849 " 3,828,841 " 3,848,658 " 3,864,973 Petry *3,921,697 *Division of No. 3,864,973 3,865,176 Dompas et al *3,955,615 *Division of No. 3,865,176 *4,155,396 " 3,878,883 Hazelett et al *3,949,805 *Division of No. 3,878,883 *3,963,068 " 3,937,270 Hazelett et al *4,002,197 *Division of No. 3,937,270 *4,062,235 " *4,082,101 " 3,937,274 Dompas 4,092,155 Dompas et al 4,150,711 Hazelett et al ______________________________________
In machines of this type, the moving belts are thin and are cooled by substantial quantities of liquid coolant, usually water containing corrosion inhibitors. This coolant withdraws heat through the casting belts and serves to cool the metal from its molten state as it enters at one end of the machine causing it to solidify as it passes through the machine.
The molten metal pushes outwardly on the belts due to metalostatic pressure or "head". Solidification of the metal product takes place from outside to inside so that, through some of its passage through the machine, it is in the form of a solidified shell having a molten, constantly decreasing, interior volume. It will also be understood that, as the metal cools and solidifies, it shrinks. The shrinkage is very slight but, nevertheless, is sufficient to cause surface regions of the metal sometimes to pull away from the moving belts or from the side dams. When this separation between areas of the metal surface and the cooling surface occurs, non-uniform cooling is caused, which results in non-uniformities in the parameters of the casing region and in non-uniformities in the cast product.
This invention in certain aspects is especially applicable to casting machines which produce ingot or slab of a width in excess of 25 inches (635 mm). Such twin-belt casting machines are generally inclined downward in use, so as to result in a head--that is, a static pressure--of liquid metal in order to fill out the casting region, i.e. the mold cavity, and to thereby press the casting belts decisively against their back-up supports. Further, by use of open- or closed-pool pouring technique, the entry of molten metal into the machine is facilitated by operating the machine at some downward incline. The aforesaid head of molten metal depends on the angle of incline, the density of the molten metal being cast, and the distance to the point of final solidification in the machine.
The force of such liquid metal head is exerted upon the casting belts and thence upon the guides or back-up supports for the belts, which we commonly call the mold back-up. Most immediately, this back-up consists of transversely disposed finned back-up rollers. These rollers and their supports have previously been made rigid in order that the ingot or slab of accurately defined and controlled gauge may be cast. The headers bearing liquid coolant can be made to serve the additional duty of providing rigid supports for the back-up rollers. Some wide machines have in their carriages central longitudinal beams or sills to lend their additional rigidity to the back-up system, for resisting the force of the molten metal to be counteracted as it presses outwardly on the wide casting belts.
The very rigidity of the above described prior art back-up means can combine with the shrinkage inherent in the freezing and cooling of the product being cast to allow air spaces to intervene between the freshly cast surface and the casting belts. These intruding spaces substantially reduce the rate of heat transfer and may render it non-uniform, with a corresponding effect on the rate and uniformity of product cooling and solidification. The reduced rate and uniformity of cooling limits the production rate, or else it requires the use of longer casting machines than would otherwise be needed.
An associated problem with the aforesaid air spaces or gaps occurring between the cast metal surface and the mold surfaces defining the casting region is the consequent degradation of the desired fine, quick-chilled crystalline structure in the cast product into coarser crystals. Such air spaces or gaps can permit the localized remelting of the cast product with consequent bleeding, or sweating of molten material from the previously cast shell itself and/or from the molten metal inside of the shell causing segregation and/or porosity in the cast product. This reheating or remelting will not occur if good mold contact is maintained.
Problems of local excess pressure can occur with a rigid mold when excess thickness is somehow frozen locally. Thus, the relatively thin casting belts will become locally overheated with a corresponding localized area of increased heat transfer due to the high localized belt pressure against the partially solidified product. Also, if a frozen piece of metal of excess thickness is inadvertently drawn into the caster, a slitting of the belt by the narrow fins of the back-up rollers or considerable damage to the precise, rigid mold back-up mechanisms can result.